1. Technical Field
The present invention relates to the field of telecommunications systems in which more than one protocol is being followed and, more particularly, to such a method and apparatus for use in such telecommunication systems supporting interworking between the Internet telephone protocol: H.323—Packet-Based Multimedia Communications Systems and the Session Initiation Protocol (SIP).
2. Related Art
The International Telecommunications Union (ITU) is an organization of the United Nations promoting the uniform provision of telecommunications services throughout the world. In furtherance of this goal and in particular, the ITU-T section issued H.323 version 1 in 1996 to describe a protocol for Internet protocol telephony applications. Meanwhile, a Session Initiation Protocol (SIP), different from H.323, has been developed since 1999 via the Internet Engineering Task Force (IETF) to also support Internet protocol telephony applications but arising from a different perspective.
In U.S. patent application Ser. Nos. 09/642,142, 09/642,279 and 09/642,298, filed Aug. 18, 2000, by Radhika R. Roy and incorporated herein by reference as to their entire contents, there is shown in the Figures and described a real-time mobility protocol, architecture and intelligent signaling scheme for computer-readable medium for real-time mobile multimedia communications and conferencing over packet-based networks. In those applications, the concept of interworking between an H.323 protocol-based network and a Session Initiation Protocol (SIP)-based network is introduced but not described in any detail. In U.S. patent application Ser. No. 09/801,914, filed Mar. 9, 2001, by Radhika R. Roy, entitled “H.323 Back-end Services for Mobility Management” and incorporated herein by reference as to its entire contents, there are described back-end services supporting mobility management.
Referring to FIG. 1, there is provided a high level comparison of H.323 and SIP showing some of the similarities and many of the differences between the two protocols. The differences result in a problem in defining how interworking, although necessary, results in difficulties to resolve. For example, not shown in FIG. 1, it may be suggested that SIP, as it has developed from a different perspective, relates more to the provision of public and free services while H.323 has evolved from the private network side and interests in financial profit. Perhaps, such a comment is not perfectly descriptive, but it may be seen from FIG. 1, that H.323, for example, is better suitable for exchange, for admission control, for policy control, for reservation of resources, encourages version compatibility and is binary-based. SIP, on the other hand, is based on text and is less complex a protocol than H.323. Consequently, there is a need, for example, to translate an address that may be in one protocol to an address based in the other protocol. Command codes and messages, too, may not have a one-for-one correspondence between one protocol and another. Nevertheless, a user utilizing an H.323 IP telephony device trying to communicate with another user utilizing an SIP-based IP telephony device or vice versa will have a need to communicate by first requesting service and then achieving a data connection to the addressed party in as efficient a manner as possible. The requested service may comprise multimedia data transfers alone or as a supplement to a conventional IP telephony voice connection.
Singh et al., in their document “Interworking Between SIP/SDP and H.323” lodged Jan. 10, 2000 with the Internet Engineering Task Force and incorporated herein by reference as to its entire contents, describe a fundamental call scenario including user registration and address resolution and call establishment. They also describe address conversion in some detail whereby, for example, text-based addresses such as kns10@columbia.edu are converted to 128.59.19.194 and the like. Also, an algorithm for finding an intersection in capability sets for handling media in terminals and users agents is described. Handling of Q.931 and H.245 messages is also described, and a detailed description of an interworking gateway behavior is provided. The document which may not comprise prior art to the present invention fails to describe, for example, how to classify messages for developing a state machine at the gateway or providing a media switching fabric for any media terminated at the gateway.
Consequently, there remains a need in the art to provide an interworking function including a state machine for interworking between two dissimilar IP telephony protocols so that users of each protocol will be able to communicate with one another.